Lion Redox Flow Battery

Redox flow batteries (RFB) are new electrical energy storage systems (the flow redox concept was developed at NASA in the 70’s) based on redox regenerative reactions converting chemical into electrical energy. Their architecture consists of a cell stack and separate rechargeable electrolytes reservoirs. The power and energy rating are decoupled: the energy capacity is a function of the electrolyte volume (and active ions concentration) while the power is a function of the surface area of the electrodes. During operation the electrolytes are pumped through the cell stack (electro chemical reactor) in which electricity is produced by a redox reaction (process in which atoms have their oxidation number changed). The two electrolytes are separated by ion permselective membranes which permit the flow of ions but prevent the mixing of the liquids. As the ions flow across the membrane an electrical current is induced in the inert electrodes. Because the RFB’s employ heterogeneous electron transfer rather than solid-state diffusion they should be more appropriately called cells than batteries. Actually, the European Patent Organization classifies redox flow cells (H01M8/18C4) as a sub-class of regenerative fuel cells (H01M8/18).

RFB’s have features which make them best energy storage solution for utility scale and renewables applications:

– They are truly closed systems with regenerative electrolytes (which do not have to be replaced).
– The inert electrodes confer exceptional cycle life (over 10,000 cycles estimated life in case of Lion).
– The recharge / discharge time ratio is close to 1/1 while in case of conventional batteries is 5 /1.
– Once charged, the flow battery’s electrolyte remains fully charged with low self-discharge.
– They are safer and more scalable than lithium-ion batteries, and can withstand extreme temperatures and periods of idleness, which makes them well suited for renewable energy applications.
– In addition to electrical recharging, flow batteries can be rapidly replenished by simply replacing the electrolyte.
– They can increase the storage capacity by increasing the size of the electrolyte tanks which makes them suited for utility scale applications. Installation costs per unit of energy decline as the system tanks grow larger
– They can cycle rapidly and deeply, without significant self discharge, have fast reaction kinetics with 1/1000s charge to discharge modes and no need for “equalization”. Due to the rapid response and the low operating costs when idle, they are well suited for voltage and frequency regulation, load leveling, spinning reserve.
– They have full to 25 % discharge while the lead acid batteries’ depth is from full to 80% state of charge.
– They have high round trip efficiencies (65 – 75%) as compared with the lead acid batteries (45%).
– They have the lowest ecological impact of all conventional energy storage technologies including hydro.
– Disadvantages of existing RFB’s are lower specific energy (making them heavy for mobile applications), low specific power (making them expensive for stationary applications) and the need for sophisticated electronics.

Other developed flow batteries include hybrid metal hydride FB. The hybrid FB use one or more electroactive components deposited as a solid layer. Proton flow batteries integrate a metal hydride storage electrode into a reversible proton exchange membrane fuel cell. Metals less expensive than lithium can be used and provide greater energy density.

Membraneless batteries employ a laminar flow in which two liquids are pumped through a channel.

R & D on Organic Redox Flow (ORF) batteries has emerged in last few years promising to overcome drawbacks preventing the deployment of existing inorganic batteries (toxic and expensive electrolyte materials). The ORF’s batteries are using sustainable organic redox active molecules, free of resources limitation and enabling unlimited combinations of anode – cathode materials (aqueous and non aqueous). ORF’s still have challenges to resolve related to energy density, cost, radical induced side reactions, electrolyte crossover, and limited life time.

Lithium-ion batteries (Li-ion or LIB) are currently popular rechargeable batteries in which lithium ions move from the negative to the positive electrode during discharging / charging. They are not only used for portable electronics (high energy density, tiny memory effect and low self-discharge) but they are becoming a common replacement in larger format applications for the traditional lead–acid batteries. The lightweight lithium-ion battery packs tend to be preferred to the heavy lead plates and acid electrolyte traditional batteries. Chemistry, performance, longevity, safety and cost vary.(lithium–sulfur LIB’s promises the highest performance / weight ratio). LIB’s can pose safety hazards since they contain flammable electrolytes and may be pressurized. Specific costs of the LIB is over $450 / Kwh. The well advertised Tesla Lithium Ion Powerwall battery (nickel – manganese – cobalt chemistry) costs over $400 / Kwh without installation hardware.

Examples of developed RFB’s are vanadium, polysulfide, bromide and uranium based. RFB’s have not proliferated yet due to low energy density (70 Wh/l compared with over 200 Wh/l for lithium iron phosphate). New research reports redox couples with higher energy density (zinc-polyiodide hydrogen-bromine and hydrogen- bromate). Even if the multi-billion dollar potential of redox flow batteries storage business is currently limited by cost and reliability, specific costs of redox flow batteries are expected to get substantially lower then those of lithium ion batteries and flow batteries are expected to become the backbone of future electricity systems.